Microwave oscillator having a delay line surrounding the interaction chamber



y 2, 68 H. LEVKOWETZ 3,391,349

MICROWAVE OSCILLATOR HAVING A DELAY LINE SURROUNDING THE INTERACTION CHAMBER Filed Sept. 27, 1965 I F/G./

United States Patent 3,391,349 MICROWAVE OSCILLATOR HAVING A DE- LAY LINE SURROUNDING THE INTER- ACTION CHAMBER Henrik Levlrowetz, Skedsmokorset, Norway, assignor to Forsvarets Forskningsinstitutt (Norwegian Research Defence Establishment), Kjeller, Norway Filed Sept. 27, 1965, Ser. No. 490,371 Claims priority, application Norway, June 18, 1965,

7 Claims. a. 331-86) ABSTRACT OF THE DISCLGSURE The invention relates to a microwave oscillator wherein a delay line attached to the walls of the interaction chamber surrounds the beam of charged particles. The charged particle beam is formed in the shape of a hollow cylinder. A radial flux is provided to cause the particles in the beam to assume a helical path. Interaction between the delay line and the spiraling beam of charged particles generates the desired electromagnetic wave.

The present invention relates to a microwave oscillator of the type comprising a circular delay line having an axially directed magnetic field and an electron gun positioned outside the delay line and directed axially of the same, the function of the oscillator being based on an interaction between a rotating electron beam and an electromagnetic wave which propagates in synchronism with the rotation of the beam on the circular delay line which encloses the electron beam and which is closed in itself.

A similar interaction relationship between a beam and a wave is present in the ordinary magnetron oscillators. In that case also, the function depends on an axially directed magnetic field which is coaxial with a circular reentrant delay line. However, in the magnetron, the electrons are emitted from a cylindrical cathode centrally positioned in the delay line and are then accelerated to synchronism with the wave by means of a radial electrostatic field and the axial magnetic field. Hereby, the electrons which are in a favorable phase relatively to the electromagnetic wave,a re supplying potential energy to the latter and consequently move in helical paths outwards towards the delay line.

Contrary to this condition, the electron emission and the subsequent acceleration in the oscillator of the present invention, are effected in an axially directed electron gun situated outside the delay line.

Prior to the injection into the central part of the delay line, the electrons are deflected by means of a radial magnetic field, to the effect that they, when in the line, and subjected to the axial magnetic field of the same, are rotating approximately synchronous with the electromagnetic wave. The electrons which then are in a favorable phase relatively to the wave, are supplying rotational energy to the same and, consequently, they will move in helical paths inwards towards the axis of the system.

For the purpose of obtaining this effect, the oscillator according to the invention is so equipped and formed as to produce, at the point at which the electron beam enters the delay line, a radially directed magnetic field having a flux which is greater than the axial magnetic flux in the delay line, to the effect that kinetic translational energy in the electron beam, supplied to the same by means of the electron gun, already at this point, i.e., even before the beam is entering the delay line, is converted into rotational energy which is then supplied to the synchronous electromagnetic wave on the delay line.

Hereby, it is important that the radial magnetic field is so formed that the electrons upon deflection, are follow- Patented July 2, 1968 "ice ing paths Which are approximately symmetrical about the axis of the delay line, as this would give the best conditions for an effective transfer of energy from beam to wave. Similarly, it is of great importance to such energy transfer, that the electrons are injected into the delay line near the inner diameter of the same, where the electric high frequency field is strong. Structurally, this may be achieved by so arranging the electron gun as to make the same generate a hollow cylindrical beam of a small thickness as compared with the radius. It is also advantageous to form the delay line similar to a conventional magnetron anode, as such a structure has been found in practice to give an effective energy transfer from the beam to the electromagnetic wave.

An oscillator according to the invention is now to be described, by way of example, in an embodiment wherein the structural principles indicated above are applied.

In the accompanying drawing, FIGURE 1 is an axial sectional view of the oscillator, and FIGURE 2 is a partial sectional view taken along the line IIII in FIGURE 1.

As illustrated in the drawing, the oscillator comprises an electron gun device K and a delay line M, separated by a partition S made of magnetically conductive material.

Both the gun part K and the delay line M are provided with separate magnet means, each generating an axially directed field in the associated part. In the drawing, such magnets are shown as being permanent magnets U1 and U2, respectively, but obviously electromagnets may also be used. The magnets are so arranged that their fluxes have opposite axial directions, their radially directed fluxes being added in the partition.

The delay line M is, generally, formed similar to an ordinary magnetron anode comprising a plurality of cavities separated by radial vanes, and provided with an output terminal P for the generated microwave power. The gun K is provided with an annular cathode E having an exterior diameter which is slightly smaller than the interior diameter of the delay line and with a positive annular anode A. The annular anode A and the partition S are provided with similar annular apertures T to provide a passage for the electron beam from the gun K to the delay line M. The electrons emitted from the cathode E are accelerated towards the anode A, thereby forming a hollow cylindrical electron beam under the focusing influence of the axial magnetic field of the magnet U1. This beam is entering the delay line through the annular apertures of the anode A and the partition S. In the annular aperture of the partition S, the electrons are encountered by the total radially directed fluxes of the two magnets U1 and U2, which will supply the electrons with a transversal impulse proportional to the flux, to the effect that the electrons are deflected from their original rectilinear paths, so as to generally make the same follow helical paths. If the fluxes of the magnets U1 and U2 are equal, the radial magnetic field in the partition S will so influence the electrons that the radius of their resultant helical paths becomes equal to the radius of the hollow cylindrical electron beam. The pitch of their paths will depend on the acceleration voltage between the anode A and the cathode E, and at a certain, critical value V this pitch becomes zero, according to the equation:

wherein e/m=the charge to mass ratio of the electron and r=the radius of the helical paths, B=the field strength of the axial magnetic field in the delay line.

This condition corresponds to a total transformation of the translational energy of the electrons rotational energy. However, for the purpose of obtaining transfer of such rotational energy to the synchronous electromagnetic wave of the delay line, it is required that the electrons maintain a certain velocity in axial direction. Consequently, the oscillator would have to be operated with an acceleration voltage V which is slightly higher than the critical value V according to the relations:

The part of the beam energy which is transformed, is then:

Jfi P, V

while the rest represents the remaining translational energy of the electrons.

Consequently, when operated with such an accelerating voltage V, the electrons are moving along helical paths with a small pitch in the direction of the axis of the delay line, and thereby stay for a comparatively long period in the regions in which interactions can occur between the electrons and the electromagnetic high frequency field of the delay line. Thus, a favorable condition is achieved for a strong coupling between the electrons and the field.

However, for the purpose of obtaining a useful transfer of energy from the electrons to the electromagnetic wave, the electrons must simultaneously rotate about the axis of the delay line in approximate synchronism with one of the electromagnetic waves capable of travelling along the delay line. If, as in the example shown in the drawing, the delay line is formed as an ordinary magnetron anode having an even number of identical cavities, each having a narrow slot facing the central interaction space, it is convenient to endeavour to excite the 1r mode of the structure, in which the phase difference between the electrical fields in subsequent cavity slots along the circumference of the interaction space is 1r radians. The phase of the wave is rotating with the same angular velocity as the electrons when the following condition is fulfilled:

wherein 1,, is the oscillation frequency of the electromagnetic field,

f is the cyclotron frequency of the axial magnetic field,

n is the number of cavities,

B is the field strength of the axial magnetic field, and

e/m is the charge to mass ratio of the electron.

The radial component of the electromagnetic high frequency field, together with the axial magnetic field usually cause an azimutal velocity modulation of the electron beam. If the synchronism conditions are approximately fulfilled, this will lead to an increased electron concentration in the regions of the interaction space, wherein the electrical field vector is of such direction that energy is transferred from the electrons to the wave. Consequently, the electrons being in such regions are gradually loosing their rotational energy and this will take place in such a way that the radius of their helical motion paths is reduced while their angular velocity is maintained to provide a favorable relationship to the Wave. However, due to their axial drift velocity they are eventually captured by the collector N.

The collector N is made of magnetically conductive material and formed similar to the partition S, in order thereby to obtain the most advantageous homogeneous magnetic field in the delay line.

In spite of the increased electron concentration in the regions wherein the electrons are capable of transferring energy to the electromagnetic wave, some electrons are nevertheless in such a position as to absorb energy from the wave. Consequently, their rotation radius is increased until they are captured by the delay line. Due to the velocity and density modulation, described above, the greater part of the electrons are, however, in a favorable phase, to the effect that as a whole energy is supplied by the beam to the wave. These conditions, i.e., the splitting of the beam in the interaction space, are indicated by the heavy black lines in FIG. 1.

The energy exchange is, as will be noted, of the same nature as that of a magnetron, and consequently, an efficiency is obtainable which is of the same order of magnitude as that of a magnetron. The present oscillator has, however, the advantage as compared with the magnetron that the cathode is situated outside the interaction space, to the effect that the cathode is not, as in the magnetron, sujected to a bombardment of misphased electrons. Such an electron bombardment may easily lead to an overheating and evaporation of active cathode material and consequently to reduced life and/or to operational disturbances. In addition hereto, the essential advantage is obtained that the present oscillator is more easily tunable, both electronically and magnetically. Small variations in acceleration voltage or in the strength of the magnetic field, will result in big variations in the axial electron velocity and, thereby in the space charge in the interaction space. Through this change in space charge, the effective resonance frequency of the cavities is shifted, and the rotation frequency of the electrons is changed, whereby the oscillation frequency is changed. A fact, which under many circumstances is highly advantageous, is that the oscillator, at the same oscillation frequency, requires an essentially weaker magnetic field than a corresponding magnetron.

I claim:

1. A microwave oscillator, comprising:

a gun for generating an axial beam of charged particles, an interaction chamber formed by cylindrically shaped delay line means located adjacent to the gun and enclosing the axial beam of charged particles,

magnetic field generating means for generating radial and axial magnetic fields, said radially directed magnetic field being confined to the region between the gun and the interaction chamber and having a flux which is greater than the flux 0f the axial magnetic field in the interaction chamber to cause a conversion of the kinetic translational energy of the charged particle beam into rotational energy to generate a synchronous electromagnetic wave by the interaction of the charged beam and the delay line means.

2. An oscillator according to claim 1 wherein the gun produces a charged particle beam in the form of a hollow cylinder.

3. An oscillator according to claim 2 wherein the gun lncludes an annular cathode having an exterior diameter which is smaller than the interior diameter of the delay line means, an annular anode spaced from said annular cathode, said annular anode having an aperture, partition means located between the gun and the interaction chamber, said partition including an aperture aligned with the aperture of the annular anode to provide passages for the charged particle beam from the gun to the delay line means in the interaction chamber.

4. The microwave oscillator according to claim 3 wherein the cylindrically shaped delay line means comprises a plurality of cavities separated by radial vanes and further including an output terminal for the generated microwave power.

5. The microwave oscillator according to claim 4 wherein the magnetic field generating means comprises a first means for providing an axial magnetic field within the gun and a radial magnetic field within the aperture of the partition, second means for generating an axial magnetic field within the interaction chamber and a second radial field within the aperture of the partition, the first axial magnetic field and the second axial magnetic field opposing one another and the first radial field and the second radial field being additive.

6. The microwave oscillator according to claim 5 wherein the radial fluxes of the first means and the second means are equal so that the radial magnetic field formed in the aperture of the partition exert a force on the charged beam such that the radius of the resultant helical paths of the charged particles is equal to the radius of the hollow cylindrical charged particle beam.

7. The microwave oscillator according to claim 6 wherein the first means consists of a magnet surrounding the gun and the second means consists of a magnet surrounding the interaction chamber.

References Cited FOREIGN PATENTS ROY LAKE, Primary Examiner.

10 I. B. MULLINS, Assistant Examiner. 

